Tracking movement of an eye within a tracking range

ABSTRACT

In certain embodiments, a system for tracking movement of an eye comprises a camera system, a computer system, and an output device. The camera system generates images of the eye. The computer system stores the images and at least one of the images as a reference image. The computer system also tracks movement of the eye within a tracking range by comparing a current image with the reference image, and by determining a movement of the eye from the comparison of the current image and the reference image. The tracking range has one or more alert points. The computer system also determines an orientation of the eye relative to at least one alert point of the tracking range. The output device outputs a range indicator that indicates the orientation of the eye relative to the at least one alert point of the tracking range.

TECHNICAL FIELD

The present disclosure relates generally to eye-tracking, and morespecifically to tracking movement of an eye within a tracking range.

BACKGROUND

Image guided systems have been developed for use with ophthalmicsurgical devices, such as cataract refractive and LASIK surgicaldevices. The systems create a digital image of the patient's eye thatcapture features of the eye, e.g., scleral vessels, limbus, and irisfeatures. This image is used to, e.g., position incisions and lensalignment in real time. The systems may have eye-tracking capabilitiesthat detect translational and rotational movement of the eye. In somesystems, eye-tracking helps keep the laser beam on target duringsurgery. Studies have shown that eye-tracking produces better outcomesand decreases complications.

BRIEF SUMMARY

In certain embodiments, a system for tracking movement of an eyecomprises a camera system, a computer system, and an output device. Thecamera system generates images of the eye. The computer system comprisesa memory and one or more processors. The memory stores the images and atleast one of the images as a reference image. The processors trackmovement of the eye within a tracking range by comparing a current imageof the plurality of images with the reference image, and by determininga movement of the eye from the comparison of the current image and thereference image. The tracking range has one or more alert points. Theprocessors also determine an orientation of the eye relative to at leastone alert point of the tracking range. The output device outputs a rangeindicator that indicates the orientation of the eye relative to the atleast one alert point of the tracking range.

In certain embodiments, a method for tracking the movement of an eyecomprises generating images of the eye. The images are stored and atleast one of the images is stored as a reference image. Movement of theeye is tracked within a tracking range by comparing a current image ofthe plurality of images with the reference image, and by determining amovement of the eye from the comparison of the current image and thereference image. The tracking range has one or more alert points. Anorientation of the eye is determined relative to at least one alertpoint of the tracking range. A range indicator that indicates theorientation of the eye relative to the at least one alert point of thetracking range is output.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure are described by way of example ingreater detail with reference to the attached figures, in which:

FIG. 1 illustrates one embodiment of a system for tracking the movementof an eye during laser treatment surgery;

FIGS. 2A and 2B illustrate examples of range indicators during differenttracking sessions;

FIGS. 3A and 3B illustrate an example of a graphical element with aslider element;

FIGS. 4A and 4B illustrate an example of a graphical element with anarrow element;

FIGS. 5A to 5D illustrate an example of a graphical element that changescolor;

FIGS. 6A to 6D illustrate an example of a graphical element that changesshape;

FIGS. 7A and 7B illustrate an example of a graphical element thatchanges size;

FIGS. 8A to 8D illustrate an example of a graphical element thatdisplays numbers;

FIGS. 9A to 9D illustrate an example of a graphical element thatdisplays words;

FIGS. 10A and 10B illustrate an example of a sound that changes infrequency;

FIGS. 11A and 11B illustrate an example of a sound that changes involume; and

FIG. 12 illustrates an example of a method for tracking the movement ofan eye that may be performed by system of FIG. 1 .

DESCRIPTION OF EXAMPLE EMBODIMENTS

Referring now to the description and drawings, example embodiments ofthe disclosed apparatuses, systems, and methods are shown in detail. Asapparent to a person of ordinary skill in the field, the disclosedembodiments are exemplary and not exhaustive of all possibleembodiments.

FIG. 1 illustrates one embodiment of a system 10 for tracking themovement of an eye 12, e.g., during a surgical or diagnostic procedure.System 10 provides a range indicator that indicates the orientation ofeye 12 relative to an alert point of the eye tracking range, so a usercan determine if eye 12 is close to moving outside of the tracking rangebefore it actually does. This can be used to avoid some undesirableeffects of moving outside of the tracking range. For example, in somesystems, if the tracking range is exceeded, the system suspendstracking, which prolongs surgery time. As another example, the systemmay report false positive tracking if the tracking range is exceeded.

In the illustrated embodiment, system 10 comprises a computer system 20,a camera system 22, a laser system 24, and an output device 26. Computersystem 20 includes one or more memories 30 and one or more processors32. In certain embodiments, camera system 22 generates images 34 of eye12. Computer system 20 controls eye tracking and laser control. Memory30 stores images 34, where at least one image 34 is stored as areference image 34 a. Processor 32 tracks eye 12 within a tracking rangeby comparing a current image 34 with reference image 34 a anddetermining movement of eye 12 (e.g., change in location and/or angularorientation) from the comparison of current image 34 and reference image34 a. The tracking range has one or more alert points near or at the endof the tracking range of system 10. Processor 32 determines theorientation of eye 12 relative to an alert point. Output device 26outputs a range indicator that indicates the orientation of eye 12relative to the alert point of the tracking range.

Camera system 22 may be any suitable system with detectors that candetect light reflected from eye 12 and generate a signal that can beused to create images 34 of eye 12. An example of camera system 22 is aCCD camera. A sequence of images 34 of eye 12 (i₁, . . . , i_(k), . . ., i_(n)) can show the movement of eye 12. An image 34 is used as areference image to compare to a current image from camera 22 to detectmovement. The reference image may any suitable image i_(k-q), q≥1 beforethe current image i_(k), e.g., the immediate previous image i_(k-1)before the current image i_(k).

Eye 12 has a location and orientation (or angular orientation or angularposition), which may be described as the configuration of eye 12. Incertain embodiments, the location of eye 12 may be expressed using (x,y, z) coordinates the coordinate system used in ophthalmologicalsurgery, where an eye feature such as an iris defines an xy-plane, andthe z-axis is the line normal to the plane an passing through a centralpoint of eye 12. For example, the location of eye 12 on the xy-plane attime t may be given by (x(t), y(t)) coordinates of a central point ofeye 12 (e.g., substantially about the pupil center or apex). Theorientation, or angular position, of eye 12 may be expressed as rotationabout a point of eye 12, which may be a central point of eye 12. Forexample, the orientation of eye 12 at time t may be given as an amounta(t) of degrees rotation away from a zero position at time t. The amountof degrees may be expressed with respect to one, two, or three axes,e.g.: a number a₁(t) of degrees rotation about an axis A₁; a numbera₁(t) of degrees rotation about an axis A₁ and a number a₂(t) of degreesrotation about an axis A₂; or a number a₁(t) of degrees rotation aboutan axis A₁, a number a₂(t) of degrees rotation about an axis A₂, and anumber a₃(t) of degrees rotation about an axis A₃. A zero position maybe an initial angular position, such as the angular position of eye 12when tracking starts at the beginning of a tracking session. A trackingsession may start, e.g., at the initialization of tracking or at arestarting of tracking after, e.g., eye 12 has moved out of the trackingrange.

Computer system 20 tracks the movement of eye 12 by determining a changein the configuration of eye 12, such as the translational and/or angular(or rotational) movement of eye 12. Translational movement is movementof all points of eye 12 by substantially the same amount insubstantially the same direction. Angular movement is movement of pointsof eye 12 about a central point of eye 12. In certain embodiments,system 20 may use image processing to locate the central point of eye12, e.g., the pupil, in the reference and current images 34 to determinetranslational movement and then translationally align the images 34using the central point. System 20 may use image processing to locatefeatures of eye 12 (e.g., blood vessels, iris features, or any otherappropriate feature) to determine angular movement and then rotationallyalign the images 34 about the central point.

FIG. 2A illustrates an example of showing the orientation (or angularposition) of eye 12. One or more lines 36 may indicate the position ofeye 12. In the illustrated example, lines 36 a-b function as axes. Lines36 a-b pass substantially through a central point 37 of pupil of eye 12and are substantially perpendicular to each other.

Referring back to FIG. 1 , in some embodiments, system 10 tracks themovement of eye 12 to be able to align an overlay image of eye 12 overimages 34 of eye 12 generated by camera system 22. Examples of anoverlay image include a diagnostic image, an image of a treatment plan,an infrared image, or other image of eye 12. The overlay image may beplaced over or blended with image 34 generated by camera system 22. Wheneye 12 moves in images 34, system 10 adjusts the overlay image tocompensate for the movement.

Computer system 20 can track the movement of eye 12 within a certaintracking range where tracking can be properly performed, e.g., performedwith suitable accuracy and/or precision. Outside of the range, system 10system may suspend tracking or may report false positive tracking. Thetracking range may be expressed as P+/−Q, where P represents a zeroposition when tracking starts and Q represents the tracking boundary,which is maximum distance away from the zero position P at whichtracking can be properly performed. The tracking range may have one ormore alert points. An alert point S is a point at which system 20provides a notification eye 12 is close to or at the tracking boundary,so S≤Q.

For tracking angular movement, the tracking range may be expressed asP+/−Q°, where P represents zero rotation when tracking starts and Qrepresents the tracking boundary. The tracking boundary Q is the maximumamount of degrees away from the zero rotation P at which tracking can beproperly performed. Q may have any suitable value. For current systems,Q is in the range of 10 to 15 degrees, such as 15 degrees. As systemsimprove, Q may be in the range of 15 to 20, 20 to 25, 25 to 30 orgreater than 30 degrees. Alert point S may have any suitable value, andmay be selected based on Q. For example, S=Q or S=Q−T, where T in therange of 1 to 5, 5 to 10, 10 to 15, 15 to 20 or greater than 20 degrees,such as S=Q−T=15−5 degrees. Alert point S may be set by system 10, ormay be set by user through user input.

As computer system 20 tracks the movement of eye 12, system 20 alsodetermines the configuration of eye 12 relative to an alert point. Forexample, system 20 determines the orientation a(t) of eye 12 relative toan alert point S. The relationship may be expressed as a differencebetween the orientation of eye 12 and the alert point, e.g., a(t)−S. Aseye 12 moves closer to the alert point S, the difference approacheszero.

Output device 26 outputs a range indicator that indicates theconfiguration (e.g., orientation and/or location) of eye 12 relative tothe alert point S of the tracking range. Output device 26 may be anysuitable device that provides computer output to a user or anothercomputer, e.g., a display, monitor, projector, speaker, headphone, orprinter. In certain embodiments, output device 26 is a display thatshows the range indicator as a graphical element. A graphical element isa visual mark of any suitable size, shape, or color that typicallyconveys information. Examples of graphical element range indicators aredescribed with reference to FIGS. 2A to 9B. In certain embodiments,output device 26 is a speaker that emits the range indicator as a sound.Examples of audio range indicators are described with reference to FIGS.10A to 11B.

Referring to FIGS. 2A and 2B, range indicator 39 indicates theorientation of eye 12 during different tracking sessions. In FIG. 2A,the zero point of the tracking session is P₁, so the tracking range ofthe session is P₁+/−Q. In FIG. 2B, the zero point of the trackingsession is P₂, so the tracking range of the session is P₂+/−Q. In FIG.2A, range indicator 39 indicates the orientation of eye 12 is within thetracking range. In FIG. 2B, range indicator 39 indicates the orientationof eye 12 is outside of the tracking range. Note the orientation of eye12 appears to be the same in both FIGS. 2A and 2B, even though eye 12 ofFIG. 2A is within the tracking range and eye 12 of FIG. 2B is not. Thisis because FIGS. 2A and 2B describe different tracking sessions, and thedetermination of whether eye 12 is within the tracking range P+/−Qdepends on the zero point P of the tracking session.

Referring back to FIG. 1 , laser system 24 receives a notification ofthe movement of eye 12, and changes a laser beam position in response tothe notification. Laser system 24 may be any suitable laser surgicaldevice that provides a laser beam to treat eye 12, and may comprise acomputer, laser source, and scanning device. In certain embodiments, thescanning device receives the notification of the movement of eye 12 andmodifies the focus of the laser beam to address or compensate for themovement. In this manner, eye movements can be taken into account eitherfor registration or for tracking purposes.

FIGS. 3A and 3B illustrate an example of a graphical element 38 with aslider element 42. In the example, graphical element 38 includes alinear element 40 and a slider element 42. Linear element 40 is a long,narrow mark that can be straight or curved. In the example, linearelement 40 represents the tracking range and has at least one alertpoint element 41 representing an alert point of the tracking range.Slider element 42 is a mark along linear element 40. In the example,slider element 42 represents the rotation of eye 12. The position of theslider element 42 relative to point 41 represents the rotation of eye 12relative to the alert point of the tracking range. FIG. 3A shows therotation of eye 12 not exceeding an alert point, i.e., is within anacceptable range. FIG. 3B shows the rotation eye 12 exceeding an alertpoint, i.e., is outside of an acceptable range.

FIGS. 4A and 4B illustrate an example of a graphical element 38 with anarrow element 46. In the example, graphical element 38 includes a linearelement 44 and an arrow element 46. Linear element 44 is similar tolinear element 40. In the example, linear element 44 represents thetracking range and has at least one alert point element 41 representingan alert point of the tracking range. Arrow element 46 is a mark thatindicates a position along linear element 44. In some cases, arrowelement 46 may look like as illustrated or may look like a compassneedle. In the example, arrow element 46 points to a point of linearelement 44 that represents the rotation of eye 12. The position wherearrow element 46 points relative to point 41 represents the rotation ofeye 12 relative to the alert point of the tracking range. FIG. 4A showsthe rotation of eye 12 not exceeding an alert point, i.e., is within anacceptable range. FIG. 4B shows the rotation eye 12 exceeding an alertpoint, i.e., is outside of an acceptable range.

FIGS. 5A to 5D illustrate an example of a graphical element 38 thatchanges color. In certain embodiments, graphical element 38 changescontinuously from a first color to a second color to indicate a changein the orientation of the eye relative to an alert point of the trackingrange. The continuous color change may be changes in shade from thefirst color to the second color. In the illustrated example, FIG. 5Ashows a green graphical element 38, indicating the rotation of eye 12 isnot close to an alert point, i.e., is within an acceptable range. FIG.5B shows a reddish green graphical element 38, indicating the rotationof eye 12 is approaching the alert point. FIG. 5C shows a greenish redgraphical element 38, indicating the rotation of eye 12 is even closerto the alert point. FIG. 5D shows a red graphical element 38, indicatingthe rotation of eye 12 has exceeded the alert point, i.e., is outside ofthe acceptable range.

FIGS. 6A to 6D illustrate an example of a graphical element 38 thatchanges shape. In certain embodiments, graphical element 38 changescontinuously from a first shape to a second shape to indicate a changein the orientation of the eye relative to an alert point of the trackingrange. The continuous change in shape may be gradual changes from thefirst shape to the second shape. In the illustrated example, FIG. 6Ashows graphical element 38 as a circle, indicating the rotation of eye12 is not close to an alert point, i.e., is within an acceptable range.FIG. 6B shows graphical element 38 as a square with rounded corners,indicating the rotation of eye 12 is approaching the alert point. FIG.6C shows graphical element 38 as an X with rounded corners, indicatingthe rotation of eye 12 is even closer to the alert point. FIG. 6D showsgraphical element 38 as an X with sharp corners, indicating the rotationof eye 12 has exceeded the alert point, i.e., is outside of theacceptable range.

FIGS. 7A and 7B illustrate an example of a graphical element 38 thatchanges size. In certain embodiments, graphical element 38 changescontinuously from a first size to a second size to indicate a change inthe orientation of the eye relative to an alert point of the trackingrange. An alert point mark 47 represents the alert point. In theillustrated example, FIG. 7A shows graphical element 38 as a bar thatdoes not cross mark 47, indicating the rotation of eye 12 does notexceed an alert point, i.e., is within an acceptable range. FIG. 7Bshows graphical element 38 as a bar that crosses mark 47, indicating therotation of eye 12 exceeds an alert point, i.e., is outside of anacceptable range.

FIGS. 8A to 8D illustrate an example of a graphical element 38 thatdisplays numbers. In certain embodiments, graphical element 38 displaysa first number 50 that continuously changes to subsequent numbers 50 toindicate a change in the orientation of the eye relative to an alertpoint of the tracking range. In the illustrated example, FIG. 8A showsgraphical element 38 displaying “3”, indicating the rotation of eye 12is not close to an alert point, i.e., is within an acceptable range.FIG. 8B shows graphical element 38 displaying “2”, indicating therotation of eye 12 is approaching the alert point. FIG. 8C showsgraphical element 38 displaying “1”, indicating the rotation of eye 12is even closer to the alert point. FIG. 8D shows graphical element 38displaying “0”, indicating the rotation of eye 12 has exceeded the alertpoint, i.e., is outside of the acceptable range.

FIGS. 9A to 9D illustrate an example of a graphical element 38 thatdisplays words. In certain embodiments, graphical element 38 displays afirst word 52 that continuously changes to subsequent words 52 toindicate a change in the orientation of the eye relative to an alertpoint of the tracking range. In the illustrated example, FIG. 9A showsgraphical element 38 displaying “OK”, indicating the rotation of eye 12is not close to an alert point, i.e., is within an acceptable range.FIG. 9B shows graphical element 38 displaying “CLOSE”, indicating therotation of eye 12 is approaching the alert point. FIG. 9C showsgraphical element 38 displaying “TOO CLOSE”, indicating the rotation ofeye 12 is even closer to the alert point. FIG. 9D shows graphicalelement 38 displaying “NO”, indicating the rotation of eye 12 hasexceeded the alert point, i.e., is outside of the acceptable range.

FIGS. 10A and 10B illustrate an example of a sound that changes infrequency. In certain embodiments, a speaker 56 emits a sound thatcontinuously changes in frequency 58 to indicate a change in theorientation of the eye relative to an alert point of the tracking range.In the example, FIG. 10A shows a sound with a frequency 58 a of x Hzindicating the rotation of eye 12 is not close to an alert point, i.e.,is within an acceptable range. FIG. 10B shows a sound with a frequency58 b of y Hz indicating the rotation of eye 12 exceeds an alert point,i.e., is outside of an acceptable range. Frequency y can be greater orless than frequency x, but not the same as frequency x.

FIGS. 11A and 11B illustrate an example of a sound that changes involume 60. In certain embodiments, a speaker 56 emits a sound thatcontinuously changes in volume 60 to indicate a change in theorientation of the eye relative to an alert point of the tracking range.In the example, FIG. 11A shows a sound with a volume 60 a of x dBindicating the rotation of eye 12 is not close to an alert point, i.e.,is within an acceptable range. FIG. 11B shows a sound with a volume 60 bof y dB indicating the rotation of eye 12 exceeds an alert point, i.e.,is outside of an acceptable range. Volume y can be greater or less thanvolume x, but not the same as volume x.

In certain cases, a speaker 56 emits words to indicate a change in theorientation of the eye relative to an alert point of the tracking range.Any suitable words may be used, e.g., the words described relative toFIGS. 9A and 9B. Other examples of words include “rotate clockwise” ifthe tracking should be rotated clockwise to avoid exceeding the trackingrange, or “rotate counter-clockwise” if the tracking should be rotatedcounter-clockwise to avoid exceeding the tracking range.

FIG. 12 illustrates an example of a method for tracking the movement ofan eye that may be performed by system 10 of FIG. 1 . In the example,the method starts at step 110, where images of the eye are generated. Atstep 112, the images are stored, and at least one image is stored as areference image. Steps 114 and 116 describe tracking eye 12 within atracking range with one or more alert points. Eye 12 is tracked at step114 by comparing a current image of the plurality of images with thereference image. Movement of eye 12 is determined from the comparison ofthe current image and the reference image at step 116. The orientationof the eye is determined relative to an alert point of the trackingrange at step 118. A range indicator that indicates the orientation ofthe eye relative to the alert point of the tracking range is output atstep 120.

A component (e.g., a computer) of the systems and apparatuses disclosedherein may include an interface, logic, and/or memory, any of which mayinclude hardware and/or software. An interface can receive input to thecomponent, provide output from the component, and/or process the inputand/or output. Logic can perform the operations of the component, e.g.,execute instructions to generate output from input. Logic may be aprocessor, such as one or more computers or one or more microprocessors.Logic may be computer-executable instructions encoded in memory that canbe executed by a computer, such as a computer program or software. Amemory can store information and may comprise one or more tangible,non-transitory, computer-readable, computer-executable storage media.Examples of memory include computer memory (e.g., Random Access Memory(RAM) or Read Only Memory (ROM)), mass storage media (e.g., a harddisk), removable storage media (e.g., a Compact Disk (CD) or a DigitalVideo Disk (DVD)), and network storage (e.g., a server or database).

Although this disclosure has been described in terms of certainembodiments, modifications (such as substitutions, additions,alterations, or omissions) of the embodiments will be apparent to thoseskilled in the art. Accordingly, modifications may be made to theembodiments without departing from the scope of the invention. Forexample, modifications may be made to the systems and apparatusesdisclosed herein. The components of the systems and apparatuses may beintegrated or separated, and the operations of the systems andapparatuses may be performed by more, fewer, or other components. Asanother example, modifications may be made to the methods disclosedherein. The methods may include more, fewer, or other steps, and thesteps may be performed in any suitable order.

What is claimed is:
 1. A system for tracking movement of an eye,comprising: a camera system configured to generate a plurality of imagesof the eye; a computer system comprising: a memory configured to storethe plurality of images and store at least one of the images as areference image; and one or more processors configured to: trackmovement of the eye within a tracking range by comparing a current imageof the plurality of images with the reference image, and by determininga rotational movement of the eye from the comparison of the currentimage and the reference image, the tracking range having a plurality ofalert points comprising a tracking boundary point and an intermediatepoint, the tracking boundary point representing a maximum number ofdegrees away from a zero point where tracking can be properly performed,the intermediate point located between the zero point and the trackingboundary point; and determine an orientation of the eye relative to atleast one alert point of the tracking range; and an output deviceconfigured to output a range indicator that indicates the orientation ofthe eye relative to the at least one alert point of the tracking range,the output device comprising a speaker that outputs the range indicatoras: a first sound if the orientation of the eye corresponds to a pointbetween the intermediate point and the tracking boundary pointindicating the eye is nearing a tracking boundary beyond which trackingcannot be performed; and a second sound different from the first soundif the orientation of the eye corresponds to a point beyond the trackingboundary point where tracking cannot be performed.
 2. The system ofclaim 1, wherein the output device comprises a display that shows therange indicator as a graphical element.
 3. The system of claim 2,wherein the graphical element comprises: a linear element representingthe tracking range, the linear element having at least one alert pointelement representing the at least one alert point of the tracking range;and a slider element corresponding to the eye, the position of theslider relative to the alert point element representing the orientationof the eye relative to the at least one alert point of the trackingrange.
 4. The system of claim 2, wherein the graphical elementcomprises: a linear element representing the tracking range, the linearelement having at least one alert point element representing the atleast one alert point of the tracking range; and an arrow elementcorresponding to the eye, the position where the arrow points relativeto the alert point element representing the orientation of the eyerelative to the at least one alert point of the tracking range.
 5. Thesystem of claim 2, wherein the graphical element changes continuouslyfrom a first color to a second color to indicate a change in theorientation of the eye relative to the at least one alert point of thetracking range.
 6. The system of claim 2, wherein the graphical elementchanges continuously from a first shape to a second shape to indicate achange in the orientation of the eye relative to the at least one alertpoint of the tracking range.
 7. The system of claim 2, wherein thegraphical element changes continuously from a first size to a secondsize to indicate a change in the orientation of the eye relative to theat least one alert point of the tracking range.
 8. The system of claim2, wherein the graphical element shows a first number that continuouslychanges to a second number to indicate a change in the orientation ofthe eye relative to the at least one alert point of the tracking range.9. The system of claim 2, wherein the graphical element shows a firstword that changes to a second word to indicate a change in theorientation of the eye relative to the at least one alert point of thetracking range.
 10. The system of claim 1, wherein the speaker emits asound that continuously changes in frequency to indicate a change in theorientation of the eye relative to the at least one alert point of thetracking range.
 11. The system of claim 1, wherein the speaker emits asound that continuously changes in volume to indicate a change in theorientation of the eye relative to the at least one alert point of thetracking range.
 12. The system of claim 1, wherein the speaker emitswords to indicate a change in the orientation of the eye relative to theat least one alert point of the tracking range.
 13. The system of claim1, wherein the image processing system receives a selection of an alertpoint of the one or more alert points from user input.
 14. The system ofclaim 1, further comprising a laser system configured to: receive anotification of the movement of the eye; and change a laser beamposition in response to the notification.
 15. The system of claim 1,wherein: the tracking boundary point is in a range of 10 to 15 degrees.16. The system of claim 1, wherein: the intermediate point is one to tendegrees away from the tracking boundary point.
 17. The system of claim1, wherein: the intermediate point is set by the system.
 18. The systemof claim 1, wherein: the intermediate point is set in response to userinput.
 19. A method for tracking the movement of an eye, comprising:generating a plurality of images of the eye; storing the plurality ofimages and at least one of the images as a reference image; and trackingmovement of the eye within a tracking range by comparing a current imageof the plurality of images with the reference image, and by determininga rotational movement of the eye from the comparison of the currentimage and the reference image, the tracking range having a plurality ofalert points comprising a tracking boundary point and an intermediatepoint, the tracking boundary point representing a maximum number ofdegrees away from a zero point where tracking can be properly performed,the intermediate point located between the zero point and the trackingboundary point and determining an orientation of the eye relative to atleast one alert point of the tracking range; and outputting a rangeindicator that indicates the orientation of the eye relative to the atleast one alert point of the tracking range, the output devicecomprising a speaker that outputs the range indicator as: a first soundif the orientation of the eye corresponds to a point between theintermediate point and the tracking boundary point indicating the eye isnearing a tracking boundary beyond which tracking cannot be performed;and a second sound different from the first sound if the orientation ofthe eye corresponds to a point beyond the tracking boundary point wheretracking cannot be performed.